Image-forming MR methods which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects and do not require ionizing radiation and they are usually not invasive.
According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a processional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collection a number of samples. A sample of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
Dynamic contrast enhanced (DCE) MRI is one of the important diagnostic cornerstones in MRI based breast cancer diagnosis. Time-resolved dynamic imaging is performed during and after the administration (iv) of contrast media (Gd) to monitor signal changes due to contrast media inflow, outflow and perfusion. In this way structural changes in vascular system (including the capillary bed) and the interstitial spaces can be visualized, helping to identify potential tumor. Partial volume effects caused by fat tissue might obscure the contrast enhancement. Therefore, currently spectral fat pre-saturation approaches are used to suppress the fat signal to improve the detectability (compare Desmond K L, et al. JMRI 2007; 25:1293).
B1-/B0-inhomogeneities hamper the quality of overall fat suppression in clinical applications. A too frequently applied chemical shift selective pre-saturation RF pulse could also contribute to SAR (specific absorption rate) limitations, especially in high-field application. Chemical shift encoding approaches, like two- and three-point-Dixon approaches as disclosed for example in Glover G H, et al. MRM 1991; 18:371, Reeder S B, et al. MRM 2004; 51:35, Reeder S B, et al. MRM 2005; 54:636-644 and Xiang Q S. MRM 2006; 56:572-584 allow separating water and fat signals in a more robust way. However, all these Dixon approaches require more data prolonging total scanning time and thus reducing temporal resolution, which is not desirable.
Multi-echo techniques (Koken et al. ISMRM Berlin 2007, 1623), measuring a number of gradient echoes after each RF excitation, could be used for Dixon encoding, but their sampling efficiency is not sufficient to compensate for the extra time needed.